1. Field of the Invention
The present invention relates to an imaging apparatus, and more particularly, to an imaging apparatus including a noise reducing function.
2. Description of the Related Art
In recent years, a technique for processing a digital signal has been developed, and thus a large amount of digital information, e.g., a moving image, a still image, sound, or the like, can be easily compressively coded so as to be recorded on a small recording medium or transmitted to a communication medium. More particularly, in the field of compression coding of an image, a standard technique for moving image coding, e.g., MPEG-2 (Moving Picture Experts Group phase 2), has become widely used, and digital video cameras using MPEG-2 has come into practical use.
An imaging function of a digital video camera includes a function referred to as a gain increase. A photographer can perform high-sensitive shooting by intentionally increasing the gain of an amplifier. However, when the gain-increased image signal is compressed with MPEG-2, image quality may be deteriorated by a random noise component.
A cause of that problem is as follows. Since random noise components in the image signal caused by the gain increase do not correlate with each other between each frame, each noise component is determined as movement of an image. Thus, a great number of coding bits are allotted to the noise. As a result of this, the number of bits which should be allotted to original image information decreases, and thus, image quality after the compression coding is largely deteriorated. To prevent the deterioration of image quality, a configuration for controlling filter processing characteristics of an image compression unit according to a gain increase is discussed in U.S. Pat. No. 5,926,224 (corresponding to International Publication No. WO/1997/005745).
Furthermore, a recent multifunction digital video camera includes a noise reduction filter, with which a photographer can set a noise elimination level so as to directly output a noise-reduced image signal to an external. Thus, the digital video camera includes a terminal adapted to directly output an image signal from a camera unit (a camera image signal) to an external image monitor, a video recording apparatus, or the like.
Since a video camera capable of outputting an image signal from a camera unit to an external includes noise reduction units both in the camera unit and an image compression unit, an excessive amount of noise reduction may be applied at the time of recording an image signal. As a result, a high-frequency component of the image signal may be lost and image quality may be deteriorated. This problem is noticeable when a photographer sets a noise reduction level in the camera unit to high and increases the gain of an amplifier.
In the configuration discussed in U.S. Pat. No. 5,926,224, a camera unit of the video camera does not include a noise reduction unit. Thus, when a camera image signal is directly output to an external with the gain increased, a noise of the camera image signal cannot be adjusted.